1. Field of the Invention
The present invention relates to a gas supplying device, for a vacuum pump for carrying out a gas sucking operation due to the transfer motion of a gas transferring body, wherein a gas, other than the process gas to be transferred by the transfer motion of the gas transferring body, is supplied to an area in which the process gas exists.
2. Description of the Related Art
In a vacuum pump disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-118286, a plurality of pairs of rotors, each meshed with another, are made to rotate. The rotational motion of the plurality of meshed rotors transfers process gas. The process gas such as perfluorocarbon (PFC) gas tends to solidify under high pressure or deteriorate due to a chemical reaction with lubricant in the pump. To solve such a problem, an inert gas such as nitrogen gas is often supplied into a passage of the process gas to dilute the same.
In Kokai No. 5-118286, an arrangement is disclosed wherein a needle valve or a fixed throttle device is provided in a passage for supplying inert gas. The needle valve is capable of adjusting a feed rate of the inert gas. The feed rate of the inert gas in a device using the fixed throttle is adjustable by selecting a proper fixed throttle device having a necessary gas feeding capacity.
When the pressure of the process gas becomes higher than that of the inert gas to be fed, the process gas may enter the supply passage of the inert gas. The ingress of the process gas may damage a device for supplying the inert gas. Thus, it is preferable to provide a check valve in the supply passage of the inert gas to prevent the waste gas from entering. This counter measure, however, is problematic in that many check valves corresponding to the number of needle valves or fixed throttle devices are necessary because each of the needle valves or fixed throttle devices requires one check valve, respectively. Accordingly, the arrangement wherein the check valves are merely provided in the supply passage of the inert gas results in an increase in the size of the vacuum pump.
An object of the present invention is to prevent the size of a vacuum pump from increasing when a function of a check valve is incorporated into a supply passage for inert gas.
For this purpose, according to one aspect of the present invention, a gas supplying device, for a vacuum pump for carrying out a gas sucking operation due to the transfer motion of a gas transferring body, supplies a first gas, different from the second gas to be transferred by the transfer motion of the gas transferring body to an area of the vacuum pump in which the second gas exists, wherein a fixed flow rate restriction means for restricting the fixed flow rate of the first gas is provided in a passage for supplying the other gas.
The fixed flow rate restriction means prevents the second gas from entering the supply passage of the first gas. The fixed flow rate restriction means is effective for suppressing an increase in the size of the vacuum pump.
According to another aspect of the present invention, there are a plurality of the supply passages, each connected to the area in which the second gas exists, and the fixed flow rate restriction means are selectively provided in the respective supply passages, for supplying proper amounts of the first gas per unit time to the area.
The fixed flow rate restriction means for supplying a proper amount of gas can be easily manufactured. The fixed flow rate restriction means is easily selectable so that a predetermined amount of gas is supplied to the area in which the second gas exists.
According to a further aspect of the present invention, a threaded hole for supplying the first gas is formed in a housing of the vacuum pump as part of the supply passage, and the fixed flow rate restriction means is screw-engaged in the threaded hole.
The arrangement, in which the fixed flow rate restriction means is attached to the housing by the screw-engagement, is simple in structure.
According to a further aspect of the present invention, the fixed flow rate restriction means is a check valve and a fixed throttle having an orifice, which is screw-engaged with the check valve wherein the check valve is disposed upstream of the fixed throttle.
The fixed flow rate restriction means including the check valve and the fixed throttle having the orifice to form a unit can be assembled by using a standardized check valve and fixed throttle available on the market.
According to still further aspect of the present invention, the vacuum pump is a multi-stage Roots pump in which a plurality of rotary shafts, each provided with rotors thereon, are arranged in parallel to each other so that the rotors on one shaft are meshed with those on the adjacent shaft, and pump chambers, each accommodating one set of the meshed rotors therein, are defined in the housing to be arranged in the axial direction of the rotary shafts.
The present invention is suitably applied to a multi-stage Roots pump.
The present invention may be more fully understood from the description of the preferred embodiments of the invention set forth below together with the accompanying drawings.